Microwave absorption study of pinning regimes in $\mathbf{Ba}(\mathbf{F}{\mathbf{e}}_{1-\mathbf{x}}\mathbf{C}{\mathbf{o}}_{\mathbf{x}}{)}_2\mathbf{A}{\mathbf{s}}_2$ single crystals

Abstract

Magnetic field dependent modulated microwave absorption (MMWA) measurements have been carried out to investigate vortex pinning effects in single crystals of the iron-based high-Tc superconductor Ba(Fe_{1-x}Co_x)_2As_2 with three different cobalt doping levels of x = 0.07, 0.09, and 0.11. The dependence of the MMWA hysteresis loops on temperature, magnetic field, and Co-concentration have been measured and analyzed using a theoretical model of microwave absorption in superconductors. The analysis reveals that in an underdoped crystal (x = 0.07) the so called \dtc-pinning due to magnetically ordered regions defines the temperature dependence of the critical current density, while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the pinning is governed by structural imperfections due to inhomogeneous distribution of cobalt dopant and has the so called δ-l character.

6 pages, 5 figures. Submitted to Phys. Rev. B

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